1. Field of the Invention
The present invention relates to a near field optical recording device, and more particularly, to a near field optical recording device having a lens overheat preventing unit at a head slider where a collective lens is mounted.
2. Description of the Background Art
As for an optical recording medium or an optical magnetic recording medium, a bit (or a recording mark) size should be minimized and a track width is to be reduced to have a high density recording capacity.
However, since the spot size of light focussed on the recording medium is restricted by a diffraction limitation, there is a limitation to improve the recording density.
For processing a large amount of data, a new optical recording method is required overcoming the limitation of the conventional optical recording method.
Recently, researches have been conducted on a near field recording and reproduction by using a near field optics, which is expected to remarkably improve the recording density.
A principle of the near field optical recording and reproduction is as follows. When light goes from layer with higher n (index of refraction) to layer with lower n, some of the light is reflected. If light progresses through a lens at an angle more than a critical angle, the light is totally reflected at the surface of the lens. In this case, due to the total reflection of the light, there is a light with a fine strength on the opposite surface of the lens, which is called evanescent wave.
Using the evanescent wave allows a high resolution which has been impossible due to the diffraction limitation of light in the existing far-field.
In the near field optical recording and reproducing optical system, light is totally reflected on a collective lens to generate an evanescent wave on the surface of the lens, and the recording and reproduction are performed by coupling the evanescent wave and the surface of recording medium.
As for an optical head of a near field optical recording device, in order to use near field light for recording, the space between an optical lens and recording medium should come within a range smaller than the wavelength of light.
The temperature on the surface of a recording medium goes up as the light is focussed during the recording and reproducing process. When the optical head nears onto the surface of the recording medium, thermal energy of the surface of the recording medium is transferred to the near field optical lens mounted on the optical head. Accordingly, the temperature of the lens being close to the surface of the recording medium goes up.
The lens of the near field optical recording device is very small and sensitive to the influence of temperature. Thus, if the lens is overheated, the light transmittance characteristic of the lens is changed due to a heat distortion of the lens, making it difficult to form an accurate beam spot.
In addition, the heat distortion affects optical characteristics (i.e., a refractivity or Numerical Aperture, etc.) of the lens, resulting in change to the recording and reproducing characteristics.
In case of the optical magnetic recording, the temperature of a recording film of the recording medium should be heated by above a curie point. Thus, the temperature of the recording film within the beam spot should be increased to above 200° C. for recording, and in case of recording of a phase change, the temperature of the recording film should be increased by above 600° C.
In case of typical optical recording devices such as CD or DVD, the temperature increase in the recording film was not a big problem because the lens is spaced apart from the recording film by about 1 mm.
In case of the near field optical recording device, however, the space between the lens and the recording film is only as far as about 50-100 nm, so that the temperature increase in the recording film causes the heat distortion to the lens and a corresponding optical characteristic change, causing a serious problem to recording and reproducing.
Consequently, in case of the near field optical recording device that the surface of the recording medium and the surface of the lens are to come very closely, it is required to restrain the temperature increase of the lens to the utmost, unlike the typical far-field recording.